Things to do after you root

Applications you can now install:
 * Wi-Fi Hotspot -- turns your phone into a 3G/4G router
 * AdFree -- edits your hosts file and blocks advertisements from applications and the web
 * Terminal Emulator -- allows you to use a Linux-style terminal on your phone
 * Titanium Backup -- creates backups of your applications that you can restore. Useful after flashing a ROM.
 * Nandroid -- By rooting you can make a complete backup of your phone, sort of like a Windows system restore point. More information

Kernels:
 * You can now flash a custom kernel that may be better for your battery life. More information

ROMS:
 * You can now completely change the operating system on your phone. You might want to do this, if, for example you aren't impressed with HTC's Sense interface. More information